The invention relates to a sheet clamping device for wrapping and fixing a sheet-shaped material around a drum.
Various apparatus utilize a sheet-shaped material which is wrapped around a drum, including a master sheet loading device in an offset printing machine, an apparatus for loading an original to be transmitted as the record paper in a facsimile system, an arrangement for mounting a sheet-shaped photosensitive material in a copying machine or the like. A variety of such apparatus have been proposed and are in practical use.
A conventional sheet clamping technique relies on a mechanical arrangement utilizing a complex combination of cams and levers, resulting in a complex mechanism and requiring a high accuracy of the parts used. Hence, an increased cost results disadvantageously. In addition to the problem of increased cost, an increased number of parts require a complex operation for the mechanism, giving rise to the likelihood of an erroneous clamping action, disengagement of a sheet from the drum and a resulting likelihood of a sheet jamming. To summarize, such arrangements do not result in a sheet clamping device having a high reliability.
In one of the sheet clamping techniques, the drum surface is formed with a multitude of holes so that air can be withdrawn into the interior of the drum, thereby clamping a sheet thereon by the partial vacuum formed within the drum. In this technique, the area of the drum on which a sheet is disposed is formed with a number of holes to permit air to be withdrawn therethrough and to attract a sheet to the drum surface in response to a negative pressure applied to the interior of the drum at the same timing as a sheet is supplied to the drum. However, the arrangement must have the capability to withdraw a large quantity of air through the holes of the suction of the air through all the holes. This often prevents a sufficient negative pressure from being building up within the drum because when the leading end of a sheet is being attracted to the drum, the remaining holes are still left open, thereby resulting in a reduced clamping action of the leading end by the air suction. As a result, a vacuum pump of an increased size must be used, leading to an increased size of the overall apparatus, an increased cost and a higher level of noise.
It is to be understood that in an apparatus in which a sheet is wrapped and fixed around the drum, such as a facsimile system, a variety of operations take place as the drum rotates, so that it is necessary that the drum undergoes a precise rotation and that such rotation be accurately detected.
Where the air suction takes place through the end of a flange on the drum in order to establish a negative pressure within the drum, a motor which drives the drum for rotation cannot be directly coupled thereto, but a transmission mechanism such as gears must be used to rotate the drum. In addition, an encoder which is used to detect the rotation of the drum cannot be directly coupled therewith. Thus, the use of such a drive transmission mechanism may cause an adverse influence upon the rotation of the drum or may prevent a precise detection of the timing of rotation. By way of example, if gears and a belt are used to rotate the drum, a nonuniform rotation may result due to backlashes or oscillations occurring in the belt. If the encoder is coupled to the drum through the gears and belt, a nonuniform rotation may often result, preventing the exact timing from being determined.
A nonuniform rotation of a drum or a failure to detect such rotation are reflected in the degradation of an image quality, which is critical in an apparatus which is used to record an image.
In a sheet clamping device of a suction type, a plurality of sheets having different sizes may be selectively retained on the drum, by providing a plurality of row of holes, each including a plurality of suction holes, along the generatrix of the drum in a manner corresponding to the various sizes of the sheets, and applying a negative pressure to a selected row or rows of suction holes. It will be seen that the number of rows of suction holes be preferably reduced as much as possible while those rows of suction holes which are to be used in common be activated together in order to facilitate the construction of a channel which introduces the negative pressure to the respective rows of such holes and their associated switching devices. However, when those rows of suction holes connected in common are associated with sheets having different lateral sizes or widths, namely, having different sizes along the generatrix of a drum as they are disposed therearound, a difficulty results in that if these suction holes are located in a region corresponding to the minimum width of the associated sheets, the lateral regions of a sheet having a greater width cannot be sufficiently attracted to the drum. On the contrary, if these suctions holes are distributed in a range corresponding to the maximum width of the sheets, a sheet having a smaller width cannot be sufficiently attracted or held to the drum as a result of a leakage of the negative pressure through those suction holes located in the axial end regions of the drum.
What is demanded of a sheet clamping device of either type is to clamp a sheet in a positive manner and in close contact with the drum surface without producing any slack therein. If part of the sheet is partly removed from the drum surface, a distance between the sheet and its associated processing mechanism such as an ink jet head of a printing machine of ink jet recording type or a read head of a facsimile system may change, preventing an exact printing or reading operation from being achieved.
In particular, in a sheet clamping device of the type in which a sheet is fed underneath a sheeting seizing claw disposed on the drum surface to have its end clamped thereby while the drum is in rotation, the sheet is conveyed with a velocity greater than the peripheral velocity of the drum in order to assure a positive abutment of the sheet against the claw, so that it is likely that the leading end of the sheet may be forced away from the drum surface.
Also, a device including a sheet seizing claw must be provided with a mechanism which reliably closes the claw whenever the leading end of the sheet is to be clamped.